Surgical tourniquet systems are commonly used to stop the flow of arterial blood into a portion of a patient's limb, thus creating a clear, dry surgical field that facilitates the performance of a surgical procedure and improves outcomes. A typical surgical tourniquet system of the prior art includes a tourniquet cuff for encircling a patient's limb at a desired location, a tourniquet instrument, and flexible tubing connecting the cuff to the instrument. In some surgical tourniquet systems of the prior art, the tourniquet cuff includes an inflatable portion, and the inflatable portion of the cuff is connected pneumatically through one or two cuff ports by flexible plastic tubing to a tourniquet instrument that includes a pressure regulator to maintain the pressure in the inflatable portion of the cuff, when applied to a patient's limb at a desired location, near a reference pressure that is above a minimum pressure required to stop arterial blood flow past the cuff during a time period suitably long for the performance of a surgical procedure. Many types of such pneumatic surgical tourniquet systems have been described in the prior art, such as those described by McEwen in U.S. Pat. No. 4,469,099, U.S. Pat. No. 4,479,494, U.S. Pat. No. 5,439,477 and by McEwen and Jameson in U.S. Pat. No. 5,556,415 and U.S. Pat. No. 5,855,589.
To achieve better overall safety and performance, and in particular to achieve greater speed and accuracy in controlling the pressure in the tourniquet cuff, some advanced tourniquet systems include tourniquet cuffs that have two separate pneumatic cuff ports, so that two separate pneumatic passageways can be established between the inflatable portion of the cuff and the tourniquet instrument, by separately connecting flexible plastic tubing between each port and the instrument. Such systems are often called dual-port tourniquet systems. In one such dual-port tourniquet system of the prior art, described in U.S. Pat. No. 4,469,099, the pneumatic pressure regulation elements within the tourniquet instrument communicate pneumatically with the inflatable portion of the cuff through one port, and a pressure sensor within the tourniquet instrument communicates pneumatically with the inflatable portion of the cuff through the second port. This configuration enables more accurate sensing, monitoring and regulation of the actual pressure in the inflatable portion of the cuff that encircles the patient's limb, in comparison to single-port tourniquet systems. In a typical single-port tourniquet system of the prior art, the tourniquet cuff has only one port and only one pneumatic passageway is established between the tourniquet cuff and the instrument. The actual cuff pressure must be sensed indirectly, through the same tubing and port that is used to increase, decrease and regulate the pressure in the cuff during surgery. As a result, in such a single-port tourniquet system of the prior art, the accuracy and speed of pressure regulation, and the accuracy of the sensed cuff pressure, are affected by the pneumatic flow resistance within the single port and within the flexible plastic tubing that pneumatically connects the port and cuff to the tourniquet instrument.
Many studies published in the medical literature have shown that the safest tourniquet pressure is the lowest pressure that will stop the flow of arterial blood past a specific cuff applied to a specific patient for the duration of that patient's surgery. Such studies have shown that higher tourniquet pressures are associated with higher risks of tourniquet-related injuries to the patient. Therefore, when a tourniquet is used in surgery, surgical staff generally try to use the lowest tourniquet pressure that in their judgment is safely possible.
It is well established in the medical literature that the optimal guideline for setting the pressure of a constant-pressure tourniquet is based on “Limb Occlusion Pressure” (LOP). LOP can be defined as the minimum pressure required, at a specific time in a specific tourniquet cuff applied to a specific patient's limb at a specific location, to stop the flow of arterial blood into the limb distal to the cuff. The currently established guideline for setting tourniquet pressure based on LOP is that an additional safety margin of pressure is added to the measured LOP, to account for physiologic variations and other changes that may be anticipated to occur normally over the duration of a surgical procedure.
Surgical staff can measure LOP manually by detecting the presence of arterial pulsations in the limb distal to a tourniquet cuff as an indicator of arterial blood flow past the cuff and into the distal limb. Such arterial pulsations can be defined as the rhythmical dilation or throbbing of arteries in the limb distal to the cuff due to blood flow produced by regular contractions of the heart. Detecting blood flow thus can be done using palpation, Doppler ultrasound or photoplethysmography to measure arterial pulsations. One technique for manual measurement of LOP based on monitoring arterial pulsations as an indicator of arterial blood flow is as follows: tourniquet cuff pressure is increased by an operator slowly from zero while monitoring arterial pulsations in the limb distal to the cuff until the pulsations can no longer be detected; the lowest tourniquet cuff pressure at which the pulsations can no longer be detected can be defined as the ascending LOP. A second manual technique is that an operator can slowly decrease tourniquet cuff pressure while monitoring to detect the appearance of arterial pulsations distal to the cuff; the highest pressure at which arterial pulsations are detected can be defined as the descending LOP. The accuracy of such manual measurements of LOP is very dependent on the sensitivity, precision and noise immunity of the technique for detecting and monitoring arterial pulsations, and on operator skill, technique and consistency. Under the best circumstances considerable elapsed time is required on the part of a skilled, experienced and consistent operator, using a sensitive and precise technique for detecting and monitoring pulsations as an indicator of distal blood flow, to accurately measure LOP by manual means.
Some surgical tourniquet systems of the prior art include means to measure LOP automatically. Prior-art tourniquet apparatus having automatic LOP measurement means are described by McEwen in U.S. Pat. No. 5,439,477 and by McEwen and Jameson in U.S. Pat. No. 5,556,415. Such prior-art systems have included blood flow transducers that employ a photoplethysmographic principle to sense blood flow in the distal limb, although other transducers have been suggested in the prior art to measure blood flow based on other principles. A blood flow transducer employing the photoplethysmographic principle uses light to indicate the volume of blood present in a transduced region, consisting of a combination of a residual blood volume and a changing blood volume resulting from arterial pulsations. An additional pressure margin based on recommendations in published surgical literature is added to the automatically measured LOP to provide a “Recommended Tourniquet Pressure” (RTP), as a guideline to help the surgical staff select the lowest tourniquet pressure that will safely stop arterial blood flow for the duration of a surgical procedure. Such prior-art systems allow the surgical staff to select the RTP, based on LOP, as the tourniquet pressure for that patient or to select another pressure based on the physician's discretion or the protocol at the institution where the surgery is being performed.
Despite their potential to recommend near-optimal settings of surgical tourniquet pressures for individual patients, prior-art surgical tourniquet systems that include means for automatic measurement of LOP have demonstrated limitations of performance that have prevented their widespread acceptance and routine use. The limitations are primarily in four areas: safety, probability of successful LOP measurement, speed of LOP measurement, and accuracy of LOP measurement.
Regarding safety, it is desirable during LOP measurement that the tourniquet cuff pressure not rise significantly above the pressure required to stop blood flow past the cuff for a significant period of time. This is because it is well established that the possibility of tourniquet-related injuries increases if tourniquet cuff pressure increases substantially. For this reason, prior-art tourniquet apparatus that measures LOP by descending from a high cuff pressure are considered to be less desirable than tourniquet apparatus that measures LOP by ascending from a low pressure. Also regarding safety, it is desirable that LOP measurements be made as quickly as possible, while still assuring that the resulting LOP measurement is sufficiently accurate to allow setting the tourniquet pressure based on the measured LOP. Speed of LOP measurement is desirable for three reasons related to safety and performance: first, it is well established that longer tourniquet times are associated with a higher possibility of tourniquet-related injuries; second, during LOP measurement, if venous outflow of blood from the limb is restricted by a pressurized tourniquet cuff for an excessively long period of time, then pooling of blood in the distal limb from arterial inflow may occur, possibly leading to passive congestion of the limb from residual blood that may be hazardous; and third, any continuing increase of residual blood in the distal limb over an extended measurement period may lead to measurement error in photoplethysmographic blood flow transducers, because such transducers inherently provide one indication of the combination of residual blood volume and varying blood volume resulting from arterial pulsations in the transduced portion, thus lengthening the time for successful completion of LOP measurement, or making successful LOP measurement impossible.
Experience with manual LOP measurement, and with prior-art tourniquet apparatus having LOP measurement capability, has shown that it is not possible in practice to measure the LOP of all patients. This is because the quality and magnitude of arterial blood flow measured by a blood flow transducer distal to the tourniquet cuff may not be sufficient in some patients for measurement or analysis, due to a variety of anatomic and physiologic factors. For such patients, the physician must revert to a standard tourniquet pressure setting based on the physician's discretion. No prior-art tourniquet system includes means to characterize the quality and magnitude of blood flow distal to the tourniquet cuff measured by a blood flow transducer, in order to quickly identify those patients and situations in which LOP measurement is unlikely to be successfully completed. As a result, considerable time may be taken in the surgical setting in an attempt to measure LOP which is ultimately unsuccessful as well as time-consuming.
Even for patients in whom LOP measurement is possible, the time required by tourniquet systems known in the prior art to successfully complete automatic LOP measurements may be considerable. In addition to the safety-related considerations described above, the extended time required for LOP measurement by prior-art tourniquet systems may significantly disrupt or delay normal activities in the operating room, and thus affect the efficiency of surgery. This is in part because the patient's operative limb must remain motionless during the measurement period, to avoid the introduction of variations in pneumatic cuff pressure and the introduction of noise due to movement of the distal blood flow transducer relative to the limb. In prior-art apparatus for measuring LOP, the reference pressure for the tourniquet cuff is typically increased from zero in many predetermined increments of increasing pressure. After each such predetermined increment or step of the reference pressure, time is required to allow the actual increased pressure within the tourniquet cuff to stabilize before measurements can be taken from the distal blood flow transducer and related to actual cuff pressure. Substantially increasing the predetermined step size in such prior-art systems might increase the speed of LOP determination, but could also decrease the accuracy of LOP measurement significantly. Thus the total time required for sufficiently accurate LOP measurement in prior-art systems can be substantial, and includes the time required to increase the reference pressure in many predetermined steps from zero, the time required to allow the actual cuff pressure to stabilize after each step, and the time required to take a measurement from the distal blood flow transducer at each step, until a LOP measurement is successfully made or until an arbitrary maximum pressure limit is reached without LOP being measured.
The accuracy of LOP measurements by prior-art tourniquet apparatus may be affected by two additional sources of error. First, because of the substantial time periods often required to measure LOP by prior-art tourniquet apparatus, error may be introduced into the LOP measurement due to accumulation of residual blood in the limb distal to the tourniquet cuff. This gradual accumulation of residual blood due to blocking of venous outflow by the tourniquet cuff can reduce the magnitude of the pulsations in blood volume that are associated with the rhythmical dilation or throbbing of the distal arteries over the duration of each cardiac cycle, from heartbeat to heartbeat. Also, such an increasing volume of residual blood in the distal limb during a measurement interval can cause a gradual change in the mean blood flow signal from a photoplethysmographic transducer during the period, for reasons described above. Such a gradual change may make valid arterial pulsations indicating arterial blood flow difficult or impossible to detect, and reduces the maximum possible amplification of the signal from the distal blood flow transducer, thus reducing the accuracy of subsequent analysis. A second source of error in LOP measurement by prior-art tourniquet apparatus results from movement of the patient's limb and movement of the distal blood flow transducer relative to the attached limb, either of which could mask valid arterial pulsations indicating blood flow or could be misinterpreted as valid arterial pulsations.
There is a need for improved surgical tourniquet apparatus for measuring LOP, to overcome the above-described limitations of prior-art tourniquet systems, so that such apparatus will be suitable for routine use in all surgical procedures involving a tourniquet. To be routinely useful in this context, apparatus for measuring LOP automatically should not introduce secondary hazards associated with the measurement of LOP, should have a high probability of successful completion after LOP measurement is initiated, should complete LOP measurement sufficiently fast so that the measurement of LOP does not disrupt or unduly delay normal activities in the operating room, and should result in an LOP measurement that is accurate within surgically acceptable expectations so that it can be used as the basis for optimal setting of tourniquet pressure. The present invention addresses the need for improved surgical tourniquet apparatus for measuring LOP.